First of all, welcome to Tug
Secondly, like others have said, take the time to learn before buying anything. A good place top start is by reading the stickies at the top of this forum; you'll learn a lot about the Marriott system. Just a brief jumping point so you know what you're reading- Marriott basically has two types of overlapping ownership: owners of weeks (legacy owners) who purchased the right to use a week every year at a specific resort, mostly floating during a specific time period, of a specific size and view category. Reservations for single week owners can be made 12 months in advance. In 2010 Marriott introduced a point system, referred to as the DC here, and with few exceptions now only sells points. Each resort is assigned a point value, depending on time of year and unit size ( as well as day of week and view). Legacy owners who have enrolled can exchange their units for points at will any given tear, getting less points than the week would rake to reserve ( referenced as skim on Tug).
Point owners, or enrolled week owners who have joined the DC system, can book wherever they want, subject to availability ( which is a biggie). Week owners who have booked weeks can either occupy their home resort or use the trading company, II, to exchange to other places, requesting a trade and hoping to get it.
Time sharing has many pluses and minuses- it enables you to vacation in a larger space with amenities such as a kitchen and laundry in many cases, because you've bought it you will tend to use it and plan vacations that you may not otherwise bother with as life intervenes, unit size is conducive to extended family travel or travel with friends, and you can save vacation money by eating in, even if only breakfast and lunches, etc.. We were blessed to have even been able to take a 4 generation trip. Over the years many of us have created priceless family memories, which many of us wouldn't trade for anything.
However, like most things, it comes at a cost- both purchase and annually. MF' s are high and forever rising. Resale weeks can be bought relatively I expensively on the resale market, but points are pricey. While MF's are still a relative bargain for most places compared to Marriott.com rentals, compared to private rentals there are some weeks that can be rented for les than the MF's.
Take your time to learn the system. I think the happiest owners in general fall into two groups- those who mostly love to return to the same place annually, such that ownership almost becomes akin to a vacation home for a week or more, and owners who like to trade a lot but have a lot of both work and family flexibility, in that they can schedules vacations relatively close to travel date and aren't tied or school schedules. Those who like to travel to different places all the time and only during set times have the most difficulty, and without a lot of planning can get frustrated. For week ownership it is best if you are able to schedule your vacation time a year ahead.
Anyway, I could go on and on, but hopefully this gives you some context to understand what is written elsewhere. Good luck, and as you proceed feel free to tell us a little about yourself and vacation goals, and you're sure to get lots of responses.